Well I don't really use it because I use my eye, but it shows the lights in a picture. left beings the darks and right being the Lights so if you have a picture that is over exposed you will see a spike on the right hand side. So you generally want to keep it in the middle. But like I said I prefer to use my eye, but I do look at it so ill go with yes

Yep started using histograms after reading how to use them in a book I purchased fairly recently, as mentioned above left is dark right is lights, if the histogram is clipped to one side you have lost details in the shadows or have blown highlights (depending on what side is clipped).

According to the book you are looking for a wide spread from left to right with a dome shaped curve, although every shot would be different so there is no perfect histogram shape as such.

I definitely feel it has helped me improve my shots as it is so much easier to get a good idea of tonal range in a shot than from purely looking at the image on the LCD.